EIGRP and AF-Interface

Within the realm of EIGRP, there are two different modes, classic and named. Classic is the usual version most people are used to where the protocol configuration starts with “router eigrp 123” or something along those lines. Named mode is similar, but configurations for it are different in syntax. Named mode may start with something like “router eigrp LAB”. From there, you must also specify the address family and the actual EIGRP AS number as well. That looks like “address-family ipv4 autonomous-system 123”. Then you could actually begin further configuration.

The af-interface default command is useful for defining user defaults to apply to EIGRP interfaces that belong to an address-family when EIGRP is configured using the named method. For example, authentication mode is disabled by default, and you can enable MD5 authentication for all EIGRP interfaces in the address-family using address-family interface configuration mode and then selectively override the new default setting using different address-family interface configuration commands.

By configuring this on the af-interface default, I am configuring it by default for all interfaces in the address-family. This is easier and faster than the past where commands needed to be entered manually on all involved router interfaces.

Overall, there are a good amount of things that you can configure on the interface level, but in EIGRP named mode, you need to do it a bit differently than in classic mode. Remembering where the configuration belongs is important. Sometime putting a command directly on an interface when it needs to be under the af-interface can result in issues or the command not working at all. My rule of thumb is “classic mode, configure the interfaces. named mode, use af-interface” No secret there or anything special. Read it, understand WHY, and just keep it in the back of your mind when configuring EIGRP.